A medical lead as referred to herein is an at least partially insulated electrical conductor that interfaces with a patient's body tissue at one end and with a detector and/or energy source at the other. A medical lead may have a covering of an open-celled porous membrane material such as expanded polytetrafluoroethylene (ePTFE). Some prior art examples of such leads are given in the following paragraphs. Where these membranes cover the electrodes of such leads, the pores of the membranes must be filled with conductive fluid such as saline or blood or other bodily fluids to conduct current through them. To facilitate filling of the pores by bodily fluids or saline, a wet-out agent may be applied to the membrane. Without such a wet-out agent, several days may be required for infusion of the pores by bodily fluids. This would delay the electrical testing of the implanted lead, requiring the patient to return for testing at a later date, and rendering the lead nonfunctional until then. On the other hand, faster wet-out time allows shorter implant time, which provides a reduced risk of infection, bleeding, and tissue dehydration. It is desired that the electrical conductivity not be altered significantly over time; that is, the impedance of the electrode system at implant should be similar to that chronically. Selection and application of a wet-out agent that can almost immediately and completely wet out ePTFE is the subject of this invention.
As used herein, the term "defibrillation" refers to either or both atrial and ventricular defibrillation.
In U.S. Pat. No. 5,090,422 to Dahl et al., which is incorporated herein by reference, a porous coating or sheath is used on an endocardial defibrillation lead to create a dissection plane with respect to adjacent tissue, to substantially prevent tissue ingrowth. Materials listed for this application include porous polyurethane and porous polytetrafluoroethylene (PTFE) used with a wetting agent or surface-modified. However, there is no disclosure of any specific wet-out agent, nor of means for its application or for modifying the surface.
In U.S. Pat. No. 5,358,516 to Myers et al., a lead assembly is disclosed having an insulated conductor covered by an ePTFE sheath. Because the ePTFE sheath is used over only the insulated conductor, but not the electrode, no means is necessary to facilitate wetting out by bodily fluids.
In U.S. Pat. No. 5,330,520 to Maddison et al., which is incorporated herein by reference, at least a portion of the outer conductor is surrounded by an outer conductive sheath formed from a suitable material having a nonabrasive effect. To insulate portions of the lead in the nonelectrode regions, pores in the conductive porous sheath are filled with a nonconductive polymer. The outer conductive sheath may be a porous polymeric sheath whose pores are infused by a conductive substance such as body fluids, a conductive gel, or a layer of poly (2-hyroxyethylmethacrylate) (polyHEMA) or Nafion.RTM. perfluorosulphonic acid. No means is disclosed for facilitating, accelerating, or enhancing wetting out of the pores by bodily fluids. According to the patent, "one particular advantage of the lead" is that "fibrous ingrowth may occur in the porous portion of the lead, thus securing the lead to the heart wall", implying that a pore size that promotes tissue ingrowth is preferred. However, this would make lead extraction difficult and is not a desirable feature for long term implantable leads.
The only method to wet out the ePTFE membrane prior to implant currently found in the literature is to briefly immerse it in an alcohol solution and then slowly extract the alcohol in a saline solution. A time-consuming step, this "alcohol approach" method is described in both Gelman and Millipore product catalogs for filtration membranes and has been successfully used in acute canine animal studies using ePTFE-covered defibrillation lead electrodes.
In the field of vascular grafts, some grafts are impregnated with gelatin or collagen (U.S. Pat. No. 5,120,833) to prevent blood leakage through them. Woven polyester (Dacron.RTM.) vascular grafts have inherently excessive porosity that requires either preclotting with the patient's own blood or preloading with a crosslinked gelatin or collagen by the manufacturer. Knitted Dacron grafts and ePTFE grafts have lower porosity that do not require preclotting steps. Because of the relatively large pore size of woven Dacron grafts compared to ePTFE, perfusion impregnation processes are simple and do not require high pressures to force liquids through the graft walls (U.S. Pat. Nos. 4,911,713 and 5,120,833). The perfusion process always includes crosslinking steps that control its dissolution rate, typically with the gelatin-loaded grafts such as Gelseal.RTM., the gelatin is claimed to completely hydrolyze in 14 days. Grafts need the gelatin in place for two weeks to allow for external tissue growth into the outer surface and, more importantly, to allow for the surface of the lumen to become completely clotted by the patient's own blood to minimize bleeding.